Pollution-related concerns, coupled with high gasoline prices, increased political tension with the worlds' largest petroleum suppliers, and increasing government support have lead to increased demand for electric vehicles. It is estimated that 10 to 12 carmakers are ready to launch plug-in models by 2015, and the electric vehicle market is expected to grow to over 2.5 million cars by the same year. Such electric vehicles include, for example, plug-in electric vehicles and plug-in hybrid electric vehicles (collectively, “PEVs”).
Unfortunately, as the number of PEVs on the road continues to increase, the risk of overloading local distribution transformers also increases. As shown in Table 1, below, recharging a single PEV may consume up to three times as much electricity as a typical home. Accordingly, overloading problems may arise when several PEVs in the same neighborhood recharge at the same time, or during the normal summer peak loads.
TABLE 1CHARGING TYPESLEVEL 1LEVEL 2LEVEL 3VOLTAGE120 VAC,240 VAC,Up to 600 VDCSingle PhaseSingle PhaseELECTRIC LOAD2 KW8 KW150 KWCHARGING TIME8-16 Hours4-8 Hours10-50 Minutes
FIG. 1 illustrates a simplified view of a basic electricity transporting grid including a power plant for generating electricity 105, a step-up transformer for stepping up voltage for transmission 110, transmission lines for carrying electricity 115, a neighborhood or substation transformer 120 for a first step down, a distribution line 125 for carrying electricity to customer premises and, finally, distribution transformers 130 for a second step down in voltage for in-premise use. In the illustrated two-tier step-down transformer configuration, the substation transformers 120 manage the load of distribution transformers 130. At each tier there is a risk of overloading (regional or local) if the dynamic behavior is not well monitored.
As shown, a distribution transformer 130 reduces the primary voltage of the electric distribution system to a utilization voltage serving customers in homes and businesses. Generally, a distribution transformer 130 may be a static device constructed with two or more windings used to transfer alternating current electric power by electromagnetic induction from one circuit to another at the same frequency but with different values of voltage and current. Distribution transformers 130 are often deployed in clusters (e.g., 3) to serve a block of homes. For example, a typical 25 kVA distribution transformer 130 serves approximately 10 homes, while a substation transformer 120 can carry 1000 s of kVA.
If one or more homes mapped to a particular distribution transformer 130 adds a particularly large load, such as a PEV, this may increase the risk of overloading the mapped distribution transformer. Further, even if a single distribution transformer 130 could handle the increased load, if a number of the distribution transformers mapped to a substation transformer 120 take on increased loads due to PEVs (or other electricity-requiring apparatuses), the cumulative effect could overload the mapped substation transformer. Accordingly, the load demands of the emerging PEV market are expected to affect the performance of the power grid on multiple levels, including the local level. This is exacerbated by the fact that transformers are typically in varied stages of useful life, with some approaching their capacity loading with the existing growth.
PEVs represent a major addition in load on a transformer. By way of example, FIG. 2 illustrates the projected impact of adding three PEVs to a single distribution transformer, where each PEV draws the equivalent of one-third the amount of power as a single home. As discussed above (see Table 1) the demand of a single PEV may be as high as about three times the load of a single home.
FIG. 2 illustrates the standard (expected) load shape L1 for a warm summer day for 8-12 smaller, older homes per 25 kVA circuit; the expected load L2 with the addition of three PEVs each at 1.4 kW charging requirement; and the expected load L3 at 3.3 kW charging requirement. As shown, even without the additional demands of the PEVs, the demand already reaches peak capacity of the transformer in the evening, when people are home from work with air conditioners and other loads using the system (see L2).
Impacts of overloading transformers in the distribution system may include: phase imbalance, power quality issues, transformer degradation and failure, as well as circuit breaker and fuse blowout as described in Ryan Liu, et al, A Survey of PEV Impacts on Electric Utilities, IEEE PES conference Jan. 17, 2011, which is incorporated herein by reference in its entirety. A presentation by Hawk Asgeirsson, P.E. of DTE Energy titled, DTE Energy DER Technology Adoption DEW analysis of Renewable, PEV & Storage, presented at the Utility/Lab Workshop on PV Technology and Systems Nov. 8-9, 2010 in Tempe Ariz. is incorporated herein by reference and provides additional background and details regarding the PEV charging challenges and limitations that the invention described herein will help to overcome.
One obvious solution that is used and contemplated in the face of increasing load demand is the replacement or upgrading of transformers to meet new and expected demand. However, mass replacements are expensive and could be prohibitively so in the case of expected PEV strain. Further, such replacements/upgrades would be wasteful in many cases, where current transformers were or would have been sufficient to meet demand. Further still, replacement/upgrades may be insufficient to meet booming demand.
Ideally, the ability to monitor and track transformer load could help facilitate targeted replacement. U.S. Pat. No. 6,711,512, which is incorporated herein by reference in its entirety, describes a system for measuring, in real time, a variety of load parameters (e.g., phase voltages, phase currents and temperatures—See Table 2 below) of a pole transformer placed on a distribution line and transferring measurements wirelessly to a central monitoring station. This system requires the addition of a sensor at the transformer. Accordingly, the transformer load data could be used to determine, based on historical load information, where a transformer (or possibly other grid equipment) might need to be upgraded or added to accommodate historical loads. However, this would not mitigate immediate transformer load concerns.
TABLE 2CommunicationUnitsAlarmLocalRemoteTransformer ParametersTank pressurepsiYYYTank vacuumpsiYYYOil temperature°YYYWinding temperature°YYYPressure relief device operationon/offYYYSudden pressure relay operationon/offYYYLiquid levelon/offYYYHydrogen gas %%NNYWater content in oil%NNYSystem ParametersFans onon/offYYYLoss of control poweron/offYYYAmbient temperature°YInput currentampsNNYInput voltagevoltsNNYOutput currentampsNNYOutput voltagevoltsNNY
With respect to PEV loads in particular, a contemplated solution for meeting increased demand is to request that home owners notify their utility of every new PEV purchase, so that the utility can (statically) retrofit the available transformer capacity over time in order to plan for peak demands. However this may not always be easy to manage, and does not consider the scenario of charging away from home. Further, load limitations discussed above may be localized to a (static) transformer and may be difficult to manage from an ultimate utility head-end without visibility to the instantaneous/accumulated load per transformer.
Accordingly, there is a need in the art for systems and methods for stabilizing a power grid to accommodate simultaneous charging of PEVs to prevent uncontrollable load on transformers. The system should be able to handle PEV charging demands that are expected to vary spatially (depending on market penetration, system configuration and socio-economics), and temporally (depending on driving patterns, battery sizes and charging connection types—see Table 1, above). Further, the system should be able to handle PEV charging impacts due to clustering (simultaneous charging at homes located on a single distribution transformer). Further still, there is a need for methods and systems that can accommodate several PEV charging scenarios, including scenarios where a PEV plugs in for an immediate charge; others where, after negotiation, a PEV may charge at a future time (e.g., hours later when rates are more affordable) at a negotiated charging rate; and still others which allow PEVs to roam between charging stations (e.g., occasionally charge away from home) and be billed to the owner's account independent of the charging location.